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Renewed Interest in Triboglide
Due to its Excellent Wear
and Friction Properties
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riboglide is a high temperature wear resistant lubricant and a derivative of NASA's 200 series
coating system. After its collaborative development in the 1990s, a thermal spray specification data sheet
was developed that covers a family of thermal sprayed high
velocity oxygen fuel (HVOF) coatings consisting of a
chrome-carbide matrix with a nickel binder and a eutectic
calcium-barium fluoride solid lubricant. The coating was
rated for use between 800°-1300°F for sliding type use and
its composition contains 70 wt% hard CrC phase in 18 wt%
Ni binder with a 12 wt% eutectic CaF2-BaF2 component.
During development, parametric studies were conducted,
where changes in spraying distance, angle, powder particle
size, feed rates, and test temperature were correlated with
tribological performance using various percentages of solid
lubricants. Initial ring-on-ring wear tests done by EG&G
Pressure Science, Beltsville, Md., on Triboglide, Inconel
718, and NiCrAlY indicate that Triboglide showed no signs
of microcracking while Inconel and NiCrAlY showed
smearing and microabrasion. Tests show that the HVOF
technique is preferred for applying Triboglide coatings.
With renewed interest from companies like Honeywell
and Boeing, Plasma Technology Inc. (PTI) is reexamining
Triboglide as a hard, wear-resistant, and low friction lubricious coating for applications such as air foil bearings, actuators, and wear components where oil-based lubrication
is scarce. Rotor shafts, actuators, and other components
that operate in high temperature and high speed environments tend to wear faster due to rapid degradation of lubricant oil, thus creating a need for solid lubrication that does
not compromise on its high temperature wear and friction
coefficient.
The HVOF thermal spray process is similar to the combustion spray process, except that HVOF was developed to
produce extremely high spray velocity. A number of HVOF
guns such as JetKote, JP 5000 or 8000, and Diamond Jet,
use different methods to achieve high velocity spraying. Jet
Kote uses a simpler system comprised of a high-pressure
combustion nozzle and air cap. Fuel gas (hydrogen, propane,
or propylene) and oxygen are supplied at high pressure, and
combustion occurs outside the nozzle but within an air cap
supplied with compressed air. The compressed air pinches
and accelerates the flame and acts as a coolant for the HVOF
gun. Powder is fed at high pressure axially from the center
of the nozzle.
Fig. 1 — Photomicrograph of PS 304 coating on a titanium
shaft.
HVOF CrC-NiCr
Triboglide
Triboglide + Ag
NASA PS 304
Fig. 2 — Cross-section photos of different Triboglide coatings and
NASA PS 304 coating.
Another method (JP 5000) consists of a high-pressure
water-cooled HVOF combustion chamber and long nozzle.
Fuel (kerosene, acetylene, propylene, and hydrogen) and
oxygen are fed into the chamber; combustion produces a hot
high-pressure flame, which is forced down a nozzle to increase its velocity. Powder may be fed axially into the
HVOF combustion chamber under high pressure or fed
through the side nozzle where the pressure is lower.
HVOF coatings are very dense, strong, and feature low
residual tensile stress or, in some cases, compressive stress,
which enables thicker coatings to be applied than previously possible with the other processes. The very high kinetic energy of particles striking the substrate surface does
not require particles to be fully molten to form high quality HVOF coatings. HVOF has become the quality standard for carbide and cermet materials, as gas velocities
over 6000 ft/s allow particle velocities greater than 3000
ft/s. The result is a coating that approaches theoretical density with bond strengths above 10,000 psi.
PTI conducted extensive wear testing of Triboglide coatings and compared them to NASA's PS 304 class coatings.
ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013
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